Can time pass in both directions at the same time?

author：China Engineering Science and Technology Knowledge Center 2021-12-07 16:54:00

Can time pass "forward" and "backward" at the same time?

According to a recent study, at least in quantum systems, the answer is yes. In their research, physicists have shown how quantum systems evolve simultaneously along two arrows of time in opposite directions (i.e., "forward" and "backward" in time).

Perhaps we need to start rethinking how we should understand and characterize the flow of time when quantum laws play a key role. The study was recently published in The Physics of Communication.

Part.1

When we look up at the stars and observe the movement of celestial bodies, we often have an eternal illusion that may make us wonder if time really exists.

Philosophers and physicists have been thinking about time for centuries. In the classical world, however, everyday experience seems to reassure us of all doubts about the existence of time: time does exist, and it is constantly passing, never to return.

This seemingly obvious comes from the fact that most macrophysical phenomena can only occur in one direction. In physics, this tendency to a single direction of time is related to the resulting entropy.

Entropy can be understood as a physical quantity that defines the degree of disorder in a system. The second law of thermodynamics tells us that the value of the entropy of an isolated system at a later moment will be greater (or at least not less than) its value at the previous moment. That is to say, the macroscopic processes of nature tend to spontaneously evolve from a state of low disorder to a state of higher disorder, and this tendency can be used to identify the direction of the arrow of time: entropy increase is the direction in which time "moves forward".

However, the microscopic laws of physics are time symmetrical, in other words, there is no so-called natural priority in the direction of time. At the most basic level, physical systems tend to follow the reversible laws of time. For a microscopic system, fluctuations obscure the direction of the arrow of time, and the flow of time is only defined on average. More specifically, in this case, we cannot infer the arrow of time, and the evolution of the system does not essentially distinguish between the so-called "forward" or "backward" time.

This difference not only poses a challenge to explaining the "passage of time", but also raises questions about the "flow of time" in the quantum field.

Part.2

Many physicists have tried to apply the idea of the thermodynamic arrow of time to the quantum realm, hoping to gain a deeper understanding of how time flows under quantum mechanisms.

One of the characteristics of the quantum world is the principle of quantum superposition, according to which when a quantum system has two possible states, then it can be in the superposition of both states at the same time.

If we extend the principle of quantum superposition to the arrow of time, we will also get that quantum systems that can be staged in both forward and backward (inversion) time directions can evolve simultaneously in both time directions.

Although this idea sounds rather absurd to our everyday experience, at its most basic level, the universal law is based on the principles of quantum mechanics. This begs the question of why we have never encountered the superposition of these temporal streams in nature.

In the new study, the scientists quantified the entropy produced by a system evolving in a quantum superposition of processes of arrows of time in two directions opposite. This often results in projecting the system into a well-defined temporal direction that coincides with the most likely of the two processes, the study found. However, when very small amounts of entropy are involved, it is physically possible to observe the result of the system evolving in both forward and backward time directions.

Part.3

This study tells us that while time is often seen as an ever-increasing parameter, in the context of quantum mechanics, the laws that govern the flow of time are much more complex.

This could mean that we need to rethink the way in which this quantity is characterized in the environment in which all quantum laws play a key role.

In addition, the researchers believe that the study also has practical implications in terms of quantum thermodynamics. For example, placing quantum systems in the superposition of alternating arrows of time may give some advantages in terms of the performance of heat machines and refrigerators.

bibliography:

https://medienportal.univie.ac.at/presse/aktuelle-pressemeldungen/detailansicht/artikel/in-quantum-mechanics-not-even-time-flows-as-you-might-expect-it-to/

https://www.bristol.ac.uk/news/2021/november/quantum-rubino-comms-physics.html

https://www.nature.com/articles/s42005-021-00759-1

Cover Source:

Aloop Visual & Science, University of Vienna, Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences

Source: Principles

Can time pass in both directions at the same time?

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